HIGH STRENGTH, HIGH STRESS CORROSION CRACKING RESISTANT AND CASTABLE Al-Zn-Mg-Cu-Zr ALLOY FOR SHAPE CAST PRODUCTS

ABSTRACT

The present invention provides an Al—Zn—Mg—Cu casting alloy that provides high strength for automotive and aerospace applications and optimized stress corrosion cracking resistance in highly corrosive and tensile environments. The inventive alloy composition includes about 3.5 wt. % to about 5.5 wt. % Zn; about 1.0 wt. % to about 3.0 wt. % Mg; about 0.5 wt. % to about 1.2 wt. % Cu; less than about 1.0 wt. % Si; less than about 0.30 wt. % Mn; less than about 0.30 wt. % Fe; and a balance of Al and incidental impurities.

RELATED APPLICATION

This application claims the benefit of and priority to U.S. Provisional Application Ser. No. 60/826,131, filed Sep. 19, 2006, the contents of which are incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

This invention relates generally to aluminum alloys for aerospace and automotive shaped castings having high tensile strength and high resistance to stress corrosion cracking (SCC).

BACKGROUND OF THE INVENTION

Cast aluminum parts are used in structural applications in automobile suspensions to reduce weight. The most commonly used group of alloys, Al7SiMg, has well established strength limits. In order to obtain lighter weight parts, higher strength material is needed with established material properties for design. At present, cast materials made of A356.0, the most commonly used Al7SiMg alloy, can reliably guarantee ultimate tensile strength of 290 MPa (42 ksi), and tensile yield strength of 220 MPa (32 ksi) with elongations of 8% or greater.

In applications where high strength is required forged products are typically used. Forged products are disadvantageously more expensive than cast products. Considerable cost savings may be realized in both automotive and aerospace applications if cast products can be used to replace forged products with little or no loss of strength, elongation performance, general corrosion resistance, stress crack corrosion resistance and fatigue strength.

A variety of alternative casting alloys exist that exhibit higher strengths than Al7SiMg alloys. However these exhibit problems in castability, corrosion performance or fluidity, which are not readily overcome. For example, U.S. patent application Ser. No. 11/111,212, titled “Heat Treatable Al—Zn—Mg—Cu Alloy for Aerospace and Automotive Castings”, filed on Apr. 21, 2005, discloses an Al—Zn—Mg—Cu alloy for shaped castings having high fatigue resistance and high strength that is suitable for automotive and aerospace applications. While the Al—Zn—Mg—Cu alloy disclosed in U.S. patent application Ser. No. 11/111,212 provides good general corrosion resistance, it has been determined that the alloy exhibits a less than optimum stress corrosion cracking (SCC) resistance in environments of high tensile stress and corrosion, which could limit it's application.

In light of the above, a need exists for a casting alloy having strengths suitable for high strength automotive and aerospace applications, while simultaneously providing high stress corrosion cracking (SCC) resistance. It is further desired that the alloy maintain acceptable levels of fatigue resistance and general corrosion resistance and castability to be suitable for providing shaped castings for aerospace and automotive applications.

SUMMARY OF THE INVENTION

Generally speaking, the present invention provides an Al—Zn—Mg—Cu alloy for shaped castings having ultimate tensile strengths greater than that achieved by comparable castings of A356, while maintaining corrosion performance suitable for automotive and aerospace applications, specifically including a good resistance to stress corrosion cracking (SCC) in severely corrosive environments of high tensile stress. Broadly, the inventive alloy is composed of

about 3.5-5.5 wt. % Zn,

about 1.0-3.0 wt. % Mg,

about 0.5-1.2 wt. % Cu,

less than about 1.0 wt. % Si,

less than about 0.30 wt. % Mn,

less than about 0.30 wt. % Fe, and

a balance of Al and incidental impurities.

In one aspect of the present invention, the stress corrosion cracking (SCC) resistance of the alloy was increased by optimizing the amount of Zn and Mg, as well as the amount of Cu. Specifically, to achieve optimized stress corrosion cracking (SCC) performance the Mg and Zn content is to he limited to less than or equal to 6.0 wt. %, and the Cu content is incorporated in greater than or equal to 0.5 wt. %. The corrosion character of the Al—Zn—Mg—Cu alloy of the present invention when in an overaged condition exhibits pitting corrosion, which is the preferred mode of corrosion in comparison to intergranular corrosion.

In another aspect of the present invention, a method of manufacturing a shaped casting is provided in which an Al—Zn—Mg—Cu alloy provides strengths greater than that achieved by comparable castings of A356, while maintaining corrosion performance that is suitable for automotive and aerospace applications, specifically including a good resistance to stress corrosion cracking in severely corrosive environments of high tensile stress. Broadly, the method includes the steps of:

preparing a molten mass of an aluminum alloy composed of:

about 3.5-5.5 wt. % Zn,

about 1-3 wt. % Mg,

about 0.5-1.2 wt % Cu,

less than about 1.0 wt. % Si,

less than about 0.30 wt. % Mn,

less than about 0.30 wt. % Fe, and

incidental impurities;

casting the melt into a cast body; and

heat treating the cast body to an overaged temper;

casting at least a portion of the melt into a mold to provide a shaped casting; and

heat treating the shaped casting to an overaged condition.

In one embodiment, the optimum conditions for the alloy to achieve high stress corrosion cracking (SCC) resistance and high tensile strength includes an alloy composed of a Mg and Zn content of 6.0 wt. % or less and a Cu content greater than 0.5 wt. % in combination with casting and heat treating to an overaged temper. In the overaged condition, the inventive Al—Zn—Mg—Cu alloy exhibits only pitting corrosion mode (general corrosion), which is the preferred corrosion mode in comparison to intergranular corrosion (IG) when tested under ASTM G110 conditions.

The inventive Al—Zn—Mg—Cu alloy and method of producing a shaped casting in addition to providing levels of strength and corrosion performance that were previously not obtainable with prior Al—Zn—Mg—Cu alloys, additionally provides acceptable hot cracking performance and fluidity for casting shaped products.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and not intended to limit the invention solely thereto, will best be appreciated in conjunction with the accompanying drawings, wherein like reference numerals denote like elements and parts, in which:

FIG. 1 is a plot illustrating the effect of Cu, Mg, and Zn on the stress corrosion cracking (SCC) resistance of an Al—Zn—Mg—Cu alloy in accordance with the present invention when subjected to a 7 day boiling salt SCC test (ASTM G 103) under a tensile stress of 240 MPa.

FIG. 2 is a plot illustrating the effect of Cu, Mg, and Zn on the Stress Corrosion Cracking (SCC) resistance of an Al—Zn—Mg—Cu alloy in accordance with the present invention when subjected to a 7 day boiling salt test (ASTM G 103) under a tensile stress of 160 MPa.

FIG. 3 is a plot illustrating the effect of Cu on the tensile yield strength of an Al—Zn—Mg—Cu alloy in accordance with the present invention.

FIGS. 4 a-4 c are photographs of test specimens formed in accordance with the present invention, and comparative examples, evaluated for general corrosion performance by immersion in a NaCl+H₂O₂ solution in accordance with ASTM G110.

FIGS. 5 a-5 c depict graphs of the depth of corrosion in microns of test specimen evaluated by ASTM G110 corrosion testing, wherein the test specimen include Al—Zn—Mg—Cu compositions including greater than 0.5 wt. % Cu and comparative examples having less than 0.5 wt. % Cu.

FIGS. 6 a-6 c are photographs of the side cross section of test specimens formed in accordance with the present invention, and comparative examples, illustrating the degree of intergranular corrosive attack.

FIG. 7 is a graph of the hot cracking index verses the Cu content of pencil probe castings of Al—Zn—Mg—Cu alloys, in accordance with the present invention.

FIG. 8 is a graph of the fluidity verses the Cu content of spiral mold castings of Al—Zn—Mg—Cu alloys, in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides an Al—Zn—Mg—Cu alloy having yield strengths, fatigue strength, general corrosion performance, and stress corrosion cracking (SCC) performance suitable for automotive and aerospace applications, while maintaining castability. All component percentages herein are by weight percent unless otherwise indicated. When referring to any numerical range of values, such ranges are understood to include each and every number and/or fraction between the stated range minimum and maximum. A range of about 0.5-1.2 wt. % Cu, for example, would expressly include all intermediate values of about 0.6, 0.7, 0.8 and all the way up to and including 1.1 wt. % Cu. As used herein, the term “incidental impurities” refers to elements that are not purposeful additions to the alloy, but that due to impurities and/or leaching from contact with manufacturing equipment, trace quantities of such elements being no greater than 0.05 wt. % that may, nevertheless, find their way into the final alloy casting or casting alloy ingot.

The Al—Zn—Mg—Cu casting alloy of the present invention is composed of:

3.5-5.5 wt. % Zn,

about 1.0-3.0 wt. % Mg,

about 0.5-1.2 wt. % Cu,

less than about 1.0 wt. % Si,

less than about 0.30 wt. % Mn,

less than about 0.30 wt. % Fe, and

a balance of Al and other incidental impurities.

In one aspect of the present invention, the Mg, Zn and Cu content of the alloy is selected to provide increased strength and stress corrosion cracking resistance. The stress corrosion cracking performance of the alloy is effected by both chemical and physical factors. From a physical standpoint, to provide an alloy having good stress corrosion cracking performance the degree of precipitation within the alloy must be of a level to provide sufficient strength, but not be so great as to cause embrittlement of the alloy. From a chemical standpoint, resistance to stress corrosion cracking requires resistance to chemical attack by corrosion.

Preferably, the Al—Zn—Mg—Cu alloy provides increased stress cracking corrosion (SCC) resistance with a composition of alloying elements and ratios including a Mg and Zn content of about 6.0 wt. % or less, and a Cu content greater than 0.5 wt. %, preferably ranging from 0.5 wt. % to 1.2 wt. %, so long as the Cu content is not so great to cause embrittlement of the alloy. Specifically, in one preferred embodiment, high tensile yield strengths on the order of about 300 MPa and Stress Corrosion Cracking resistance at a stress of up to 240 MPa may be provided by the Al—Zn—Mg—Cu alloy, in accordance with the present invention.

In one aspect of the present invention, the Mg and Zn content of the Al—Zn—Mg—Cu alloy is preferably selected to provide strength enhancing MgZn₂ precipitates. Preferably, the Zn/Mg ration is about 3.3 or less and the total of the Mg and Zn content is less than about 6.0 wt. % of the alloy composition. During processing of the Al—Zn—Mg—Cu alloy of the present invention, Mg and Zn from the alloy precipitate to form MgZn₂ in the Al Matrix and at the grain boundary. Increasing the Mg and Zn content to greater than 6.0 wt. % may produce excess MgZn₂ at the grain boundaries, resulting in reduced stress corrosion cracking (SCC) resistance. In one preferred embodiment, the Zn is at a concentration of about 3.8 to 4.6 wt. %, most preferably ranging from 4.0 wt. % to 4.4 wt. %. In one preferred embodiment, the Mg is at a concentration of concentration of about 1.2 wt. % to 1.8 wt. %, most preferably ranging from 1.4 wt. % to 1.6 wt. %.

In another aspect of the present invention, the Cu content of the Al—Zn—Mg—Cu alloy is select to substantially reduce intergranular colTosion, while further providing precipitate hardening mechanisms through the formation of Al₂Cu (θ-phase) precipitates or Al₂CuMg (s-phase) precipitates. Preferably, the Cu content is selected to increase the corrosion potential of the precipitate free zone of the alloy relative to the alloy's aluminum matrix. In one embodiment, the Cu content is greater than about 0.5 wt. %, preferably ranging from about 0.5 wt. % to about 1.2 wt. %, even more preferably ranging from about 0.65 wt. % to about 1.0 wt. % and most preferably ranging from about 0.7 wt. % to 0.8 wt. %. Increasing the Cu to greater than about 1.2 wt. % may result in an excess of constituent particles of Al—Fe—Cu at the grain boundary, which may decrease the alloy's ductility, fatigue resistance and toughness.

The Al—Zn—Mg—Cu alloy of the present invention provides increased stress corrosion cracking (SCC) resistance, wherein in one aspect the Mg, Zn and Cu constituents are selected to substantially reduce intergranular corrosion. As opposed to general corrosion or pitting, intergranular corrosion can be particularly troublesome as occurring below the surface of the casting, wherein severe corrosion may occur without any indication by visual inspection. Additionally, localized corrosion at the grain boundaries by intergranular corrosion accelerates failure, as opposed to the more homogenous corrosion provided by pitting.

Intergranular corrosion in prior Al—Zn—Mg alloys results from differences in the corrosion potential between the Aluminum Alloy Matrix, the precipitate free Zone (PFZ) and the grain boundary of the casting alloy. The differences in corrosion potential in prior alloys results from the incorporation of the elements that are at least in part introduced for precipitate hardening mechanisms, such as Mg and Zn.

Precipitation at the grain boundary after quench is initially continuous, since diffusion at the grain boundary is very fast relative to diffusion within the Al Matrix due to the grain boundary's open structure. Therefore, precipitates such as MgZn₂ more readily form large continuous precipitates at the grain boundary, whereas the Al matrix restricts diffusion and growth of precipitates resulting in a more uniform fine dispersion of precipitates. At the interface of the Al matrix and the grain boundaries is the precipitate free zone (PFZ), being substantially free of precipitates, in which the alloying elements, such as Mg and Zn, are in solution.

In one instance, intergranular corrosion results from the equivalent of a galvanic cell (micro-galvanic corrosion) formed between the Aluminum Matrix and the precipitate free zone (PFZ) due to the potential difference between the differing composition of the aluminum matrix and the composition of the precipitate free zone (PFZ). In prior Al—Zn—Mg alloys the corrosion potential difference between the matrix and the precipitate free zone (PFZ) can be significant, in which the casting is particularly susceptible to intergranular corrosion and typically degrades. Such degradation can results in decreased resistance to stress corrosion cracking and premature failure of the casting. In prior Al—Zn—Mg alloys the corrosion potential difference between the Aluminum Matrix and the precipitate free zone (PFZ) typically results from elements of Mg and Zn incorporated into the PFZ solution, wherein the incorporation of Mg and Zn decreases the corrosion potential of the precipitate free zone (PFZ) relative to the Al matrix.

In one aspect of the present invention the Cu content is selected to increase the corrosion potential of the precipitate free zone (PFZ) relative the Al matrix. Preferably, the incorporation of Cu into the alloy, and hence the precipitate free zone, offsets the decrease in corrosion potential resulting from the Mg and Zn incorporated in the metal solution at the precipitate free zone, preferably to provide uniformity in corrosion potential between the precipitate free zone and the aluminum matrix. The alloy of the present invention by specifying and controlling the alloying amounts maintains a balance between the electrochemical potential of the Al matrix and the precipitate free zone. By employing an alloy chemistry that reduces or eliminates the potential difference between the precipitate free zone and the Al matrix, the present invention increases stress corrosion cracking resistance in one aspect by significantly reducing or eliminating the localized corrosion.

The alloy of the present invention may further include up to about 1.0 wt. % Silicon, wherein Si may improve castability. Further, lower levels of Si may be employed to increase strength. For some applications, manganese in amounts up to about 0.3 wt. % may be employed.

The alloy may also contain grain refiners such as titanium diboride, TiB₂ or titanium carbide, TiC and/or anti-grain growth agents such as zirconium, manganese or scandium. If titanium diboride is employed as a grain refiner, the concentration of boron in the alloy may be in a range from 0.0025 wt. % to 0.05 wt. %. Likewise, if titanium carbide is employed as a grain refiner, the concentration of carbon in the alloy may be in the range from 0.0025 wt. % to 0.05 wt. %. Typical grain refiners arc aluminum alloys containing TiC or TiB₂.

Zirconium, if used to prevent grain growth during solution heat treatment, is generally employed in a range below 0.2 wt. %, preferably ranging from 0.05 wt. % to 0.2 wt. %.

Scandium may also be used in a range below 0.3 wt. %, preferably ranging from 0.05 wt. % to 0.3 wt. %.

In another aspect of the present invention, a heat treatment in conjunction with the alloying elements and ratio's optimizes precipitation at the grain boundaries to increase stress corrosion cracking resistance. Precipitates, such as MgZn₂ and Cu precipitates, form within the metal matrix and at the grain boundary. The precipitates at the grain boundary are susceptible to corrosive attack. Moreover, precipitation at grain boundary after alloy quench initially includes a continuous distribution of fine precipitates and disadvantageously results in localized continuous corrosion at the grain boundary. Corrosion of the continuous and fine precipitates at the grain boundary (intergranular corrosion) disadvantageously decreases the alloy's stress corrosion cracking (SCC) resistance.

The present invention provides a heat treatment to overage the alloy, wherein the heat treatment results in coarsening of the fine precipitates at the grain boundary to provide a discontinuous distribution of large precipitates interrupted by Aluminum. Aluminum has a greater resistance to corrosion than the grain boundary precipitates. Therefore, the discontinuous distribution of large precipitates at the grain boundary results in a discontinuous mode of corrosion at the grain boundary, which advantageously increases the stress crack corrosion resistance (SCC) of the alloy.

For the purposes of this disclosure the term “overage” or “overage temper” or “overaged condition” denotes that the time and temperature of the heat treatment is selected to sacrifice a degree of strength from the alloy's peak strength for improved stress corrosion cracking (SCC) resistance. Applicants state that the term “peak strength” or “peak condition” denotes the maximum tensile strength or yield strength that may be achieved for a given precipitate hardening composition, such as, but not limited to, Al—Zn—Mg—Cu alloy system, wherein the strength is dependent on the temperature and time of the heat treatment.

Preferably, the heat treatment to be used with the Al—Zn—Mg—Cu alloy including a Mg and Zn content of 6.0 wt. % or less, and a Cu content greater than 0.5 wt. % to provide increased stress corrosion cracking performance includes at least one treatment at a temperature of greater than 340° F., preferably ranging from 340° F. to 380° F., for a time period of 4.0 hours or greater. In one preferred embodiment, the heat treatment includes two stages. In a first stage the casting is heated from room temperature to 250° F. within a time period of one hour and heated from 250° F. to greater than 340° F. within a time period of one hour. In a second stage, the alloy is aged at greater than 340° F. until achieving an overaged temper, wherein the second stage is conducted for greater than four hours.

In the overaged temper, the Al—Zn—Mg—Cu alloy system demonstrates 50% higher tensile yield strength than is obtainable from A356.0-T6, while maintaining similar elongations and providing stress corrosion cracking (SCC) resistance at a stress of up to 240 MPa and is applicable to part designs requiring higher strength than AlSiMg alloys that are readily available today, such as A356.0-T6 or A357.0-T6. Fatigue performance in the T6 temper is increased over the A356.0-T6 material by 45%. Specifically, high tensile strengths on the order of about 300 MPa and stress corrosion cracking (SCC) resistance at a stress of up to 240 MPa are provided by the Al—Zn—Mg—Cu alloy of the present invention.

In addition to providing increased resistance to stress corrosion cracking (SCC) and providing strengths suitable for automotive castings, the alloy of the present invention provides acceptable general corrosion (pitting) performance. Further, the castability of the inventive Al—Zn—Mg—Cu alloy is suitable for providing shaped castings.

Although the invention has been described generally above, the following examples are provided to further illustrate the present invention and demonstrate some advantages that arise therefrom. It is not intended that the invention be limited to the specific examples disclosed.

Table 1 includes alloy compositions (Alloy composition numbers 1-17) having Mg, Cu, and Zn in accordance with the present invention and includes the composition of comparative examples. Alloy composition numbers 1-5 represent some embodiments of the alloy of the present invention having about 3.5 wt. % to about 5.5 wt. %. Zn, about 1.0 wt. % to about 3.0 wt. %. Mg. and about 0.5 wt. % to about 1.2 wt. % Cu, wherein the total Mg and Zn content ranges about from 5.2 wt. % to about 5.7 wt. %, the Zn/Mg ratio ranges from about 2.66 wt. % to about 3.75 wt. %, and the Cu content ranges from about 0.65 wt. % to about 0.85 wt. %. Alloy composition numbers 6-9 and 18-20 represent comparative examples of alloys in which the Cu content is less than 0.5 wt. %. Alloy composition numbers 10-13 represent comparative examples of alloys in which the Mg and Zn content is equal to 6.0 wt. %. Alloy composition numbers 14-17 represent comparative examples of alloys in which the Mg and Zn content is greater than 6.0 wt. %. In each of the compositions listed in Table 1, the Si content is less than 0.05 wt. %, the Fe content is less than 0.05 wt. %, the Mn content is less than 0.05 wt. %, the Zr content is less than 0.09 wt. %, the B content is less than 0.02 wt. %, and the Ti content is less than 0.06 wt. %.

TABLE 1 ALLOY COMPOSITION Alloy Composition Alloy # Zn Mg Cu Mg + Zn Zn/Mg 1 4 1.2 0.85 5.2 3.3 2 4 1.5 0.65 5.5 2.7 3 4 1.5 0.85 5.5 2.7 4 4.5 1.2 0.65 5.7 3.8 5 4.5 1.2 0.85 5.7 3.8 6 4 1.2 0 5.2 3.3 7 4 1.2 0.35 5.2 2.7 8 4 1.5 0.35 5.5 2.7 9 4.5 1.2 0.35 5.7 3.8 10 4.5 1.5 0.25 6 3 11 4.5 1.5 0.45 6 3 12 4.5 1.5 0.65 6 3 13 4.5 1.5 0.85 6 3 14 4.5 1.8 0.25 6.3 2.5 15 4.5 1.8 0.45 6.3 2.5 16 4.5 1.8 0.65 6.3 2.5 17 4.5 1.8 0.85 6.3 2.5 18 4 1.2 0.25 5.2 3.3 19 4 1.5 0 5.5 2.7 20 4.5 1.2 0 5.7 3.8

Stress Corrosion Cracking Resistance

Tables 2-4 provide the results of boiling salt stress corrosion cracking testing for test samples having the alloy compositions listed in Table 1, wherein the boiling salt stress corrosion cracking testing under stress levels of 160 MPa (representative of 50% of the alloy's tensile yield strength target) and 240 MPa (representative of 75% of the alloy's tensile yield strength) was conducted in accordance with the “Standard Practice for Evaluating Stress-Corrosion Cracking Resistance of Low Copper 7XXX Series Al—Zn—Mg—Cu Alloys” as described in ASTM G103. In accordance with the guidelines of ASTM G103, stressed specimens are totally and continuously immersed in boiling solution containing about 6% sodium chloride for up to 168 hrs. The specimens are regularly checked for visual cracking. The time to failure is used to indicate the stress corrosion cracking (SCC) resistance of the aluminum alloys. A test specimen was considered to have acceptable stress corrosion cracking (SCC) resistance if it could survive the boiling salt test for a time period of 96 hours. The boiling salt stress corrosion cracking (SCC) test was conducted for seven days, wherein test samples that did not fail during the seven day period were given a value of 168 hours.

Table 2 includes the stress corrosion cracking (SCC) data provided for Al—Zn—Mg—Cu alloys (alloy composition numbers 1-5) having alloying ranges within the scope of the present invention and heat treated to an overaged condition. The alloy heat treatment included two stages, in which the first stage included heating the alloy from room temperature to 250° F. within one hour. The second stage is aging the alloy to the overaged condition, wherein Table 2 includes data for aging at 340° F. for 16 hours, and aging at 340° F. for 24 hours.

TABLE 2 BOILING SALT STRESS CORROSION CRACKING TEST 2nd state aging at 340 F. for 16 hrs 2nd state aging at 340 F. for 24 hrs Time (hours) at Time (hours) at Time (hours) at Time (hours) at Alloy # 160 Mpa 240.0 MPa 160 Mpa 240.0 MPa 1 168 168 168 168 168 168 168 168 168 132 168 168 2 168 168 168 107 168 168 168 168 168 132 166 168 3 168 168 168 168 168 168 168 168 168 168 168 168 4 168 168 168 166 168 168 168 168 168 58 132 168 5 168 168 168 107 168 168 168 168 168 168 168 168

As indicated by Table 2, the Al—Zn—Mg—Cu alloys having alloying ranges within the scope of the present invention (Alloys #1-5) and heat treated to an overaged condition survived at least 96 hours of stress corrosion cracking (SCC) testing in accordance with the boiling salt test. Generally, the test specimens survived from 119 hours to the entire length of the test (168 hours).

SCC testing was not conducted for test specimens of alloy composition numbers 1-9 being treated with a second stage heating step of 340° F. for four hours, since the intergranular corrosion of these tests specimens as measured using the Standard Practice for Evaluating Intergranular Corrosion Resistance of Heat Treatable Aluminum Alloys by Immersion in Sodium Chloride+Hydrogen Peroxide Solution in accordance with ASTM G110 indicated that this period of aging was not suitable to provide sufficient SCC resistance.

Table 3 includes the stress corrosion cracking (SCC) data provided for Al—Zn—Mg—Cu alloys (alloy composition numbers 6-9) similar to the alloy of the present invention except for having a Cu content of less than 0.5 wt. %. Alloy composition numbers 6-9 were tested for stress corrosion cracking (SCC) resistance using the same boiling salt test applied to alloy composition numbers 1-5. Alloy composition numbers 6-9 where aged using a heat treatment that includes two stages, in which the first stage included heating the alloy from room temperature to 250° F. within one hour. The second stage is aging the alloy to the overaged condition, wherein Table 2 includes data for aging at 340° F. for 4 hours, and aging at 340° F. for 16 hours.

TABLE 3 BOILING SALT STRESS CORROSION CRACKING TEST FOR AlZnMgCu ALLOY HAVING LESS TI IAN 0.5 wt. % Cu 2nd state aging at 340 F. for 4 hrs 2nd state aging at 340 F. for 16 hrs Time (hours) at Time (hours) at Time (hours) at Time (hours) at Alloy # 160 Mpa 240.0 MPa 160 Mpa 240.0 MPa 6 2 2 90 4 4 4 7 4 5 139 4 4 24 44 168 168 18 24 168 8 70 168 168 5 90 125 168 168 168 18 44 72 9 18 148 168 4 18 168 18 72 168 4 18 18

As indicated in Table 3, alloy composition numbers 6-9 having a Cu content of less than 0.5 wt. % displayed a high incidence of failure before reaching 96 hours of under stress at 160 MPa or 240 MPa under boiling salt testing. Specifically, only one test specimen having less than 0.5 wt. % Cu and aged for 16 hours at 340° F. passed the boiling salt corrosion test under a stress of 240 MPa, representing 75% of the desired minimum yield strength. Typically, alloy composition numbers 6-9 failed within 4-72 hours of testing under boiling salt test.

Table 4 includes the stress corrosion cracking (SCC) data provided for Al—Zn—Mg—Cu alloys (alloy composition numbers 10-14) similar to the alloy of the present invention except for having a combined Zn and Mg content of 6.0 wt. % or greater. The alloy heat treatment included two stages, in which the first stage included heating the alloy from room temperature to 250° F. within one hour. The second stage includes aging the alloy to the overaged condition, wherein Table 4 includes data for aging at 340° F. for 4 hours, 340° F. for 16 hours, and aging at 340° F. for 24 hours.

TABLE 4 BOILING SALT STRESS CORROSION CRACKING TEST FOR AlZnMgCu ALLOY HAVING AT LEAST 6.0 wt. % Mg AND Zn. 2nd stage aging at 340 F. for 2nd stage 4 hrs aging at 340 F. for 16 hrs 2nd state aging at 340 F. for 24 hrs Alloy # 160 Mpa 160 MPa 240.0 MPa 160 Mpa 240.0 MPa 10 2 2 3 11 3 3 10 3 5 29 12 20 20 20 168 168 168 24 24 24 168 168 168 21 58 168 13 168 168 168 168 168 90 68 90 168 68 114 168 58 76 168 14 1 1 2 15 1 2 3 5 10 10 16 20 4 20 60 168 17 44 44 168

As indicated in Table 4, alloy composition numbers 10-17 having a total Mg and Zn content of 6.0 wt. % or greater displayed a high incidence of failure before being subjected to 96 hours of stress at 160 MPa or 240 MPa under boiling salt SCC testing. As compared to stress corrosion cracking (SCC) performance of alloy composition numbers 1-5 having a total Mg and Zn content of less than 6.0 wt. % illustrated in Table 1, alloy composition numbers 10-17 having a total Mg and Zn or 6.0 wt. % or greater disadvantageously exhibited reduced stress corrosion crack resistance.

Increasing the Mg and Zn content to 6.0 wt. % or greater introduces an excess of MgZn₂ to the alloy, wherein the excess MgZn₂ decreases the chemical potential at the precipitate free zone (PFZ) relative to the alumina matrix to a level that can not be offset by the incorporation of Cu, without increasing the amount of ALCuFe and AlCuFeSi at the grain boundary, which disadvantageously reduces the alloys fracture toughness. Specifically, Alloy composition numbers 14-17 having a total Mg and Zn content of 6.3 wt. % exhibited decreased stress corrosion cracking (SCC) resistance than alloy composition numbers 10-13 having a total Mg and Zn content of 6.0 wt. %.

The data included in Tables 1-4 has been plotted in FIGS. 1 and 2. FIG. 1 illustrates the alloy compositions that pass the 96 hour boiling salt water stress corrosion cracking (SCC) resistance test under a stress of 240 MPa (˜75% of the alloy's tensile yield strength target), hence providing suitable stress corrosion cracking (SCC) resistance. FIG. 2 illustrates the alloy compositions that pass the 96 hour boiling salt water stress corrosion cracking (SCC) resistance test under a stress of 160 MPa (˜50% of the alloy's tensile yield strength target), hence providing suitable stress corrosion cracking (SCC) resistance. Referring to FIGS. 1 and 2, reference lines 10 a, 10 b represents 96 hours of boiling salt SCC testing, wherein the area 15 a, 15 b under the curve presented by reference line 10 a, 10 b indicates alloy compositions having suitable stress corrosion cracking (SCC) resistance.

To provide sufficient stress crack resistance to pass boiling salt SCC testing an Al—Zn—Mg—Cu alloy requires that the total Mg and Zn content be less than 6.0 wt. % and the Cu content be greater than 0.5 wt. %, and that the alloy be treated to an overaged condition, preferably including greater than four hours aging at 340° F., and even more preferably including 16 hrs of aging at 340° F.

Mechanical Properties

Tables 5-7 provide the results of mechanical testing for test samples having the alloy compositions listed in Table 1, wherein the mechanical properties measured included tensile yield strength (TYS), ultimate tensile strength (UTS) and percent elongation (E). Similar to the stress corrosion cracking (SCC) evaluation, each test sample was treated to a two-step heat treatment was used, in which the first stage including keeping the heat treatment constant at 250° F. for 3 hours. Following the first stage, an aging stage was conducted, in which the furnace temperature was raised to 340° F. for soaking times ranging from 4 to 32 hours. The alloy reached peak strength at about 4 hours at 340° F. Overaged conditions were investigated at 16 hours, and 24 hours at 340° F. Test specimens were considered to have acceptable mechanical properties when providing tensile yield strength (TYS) on the order of at least 300 MPa, wherein at the lab scale, test specimens having a tensile yield strength (TYS) being on the order of 320 MPa, were highly preferred.

Table 5 includes the mechanical properties measured for Al—Zn—Mg—Cu alloys (alloy composition numbers 1-5) having alloying ranges and ratios within the scope of the present invention and heat treated to an overaged condition.

TABLE 5 Ultimate Yield Tensile 2^(nd) step aging Strength Strength time, hrs ALLOY # (MPa) (MPa) Elongation % @340 F. 1 322.5 377.0 11.0 16.0 1 312.0 370.0 11.0 24.0 2 326.0 381.5 12.0 16.0 2 317.0 372.0 12.0 24.0 3 325.0 377.5 10.0 16.0 3 328.0 381.5 12.0 24.0 4 295.5 339.5 16.0 4.0 4 315.0 368.5 12.0 16.0 4 307.0 362.3 12.0 24.0 5 315.0 372.0 13.5 16.0 5 306.0 364.0 14.0 24.0

As indicated by Table 5, the Al—Zn—Mg—Cu alloys having alloying ranges within the scope of the present invention (Alloys #1-5) and heat treated to an overaged condition provided a tensile yield strength (TYS) on the order of at least 300 MPa. In a preferred embodiment, the Zn/Mg ratio is preferably less than ˜3.0, since increasing the Zn/Mg ratio for a fixed amount of Zn+Mg to greater than 3.0 typically results in a reduction of MgZn2 strengthening precipitates disadvantageously reducing tensile yield strength.

For example, alloy composition numbers 1-3 having Zn/Mg rations ranging from 2.77 to about 3.0 have a higher tensile yield strength than alloy composition numbers 4-5 having a Zn/Mg ratio being greater than 3.0. as illustrated in Table 5. The lab scale test specimens having a Zn/Mg ratio from 2.77 to 3.0 (alloy composition numbers 1-3) provided a tensile yield strength (TYS) being on the order of 320 MPa or greater, whereas test specimen having a Zn/Mg ratio on the order of 3.3 recorded lower tensile yield strength (TYS) values being, in some instances being closer to 300 MPa.

As discussed above, the alloy of the present invention includes greater than 0.5 wt. % Cu to substantially minimize the effect of the Mg and Zn on the difference in corrosion potential between the Al matrix and the precipitate free zone (PFZ) to provide an alloy having increased SCC resistance, while maintaining tensile properties suitable for high strength applications. Table 6 illustrates that the increased Cu content of the Al—Zn—Mg—Cu alloys of the present invention has a minimal effect on the alloy's tensile properties when compared to alloys having lower Cu contents. Specifically, Table 6 includes the tensile properties measured for Al—Zn—Mg—Cu alloys (alloy composition numbers 6-9 and 18-20) having a Cu content of less than 0.5 wt. %.

TABLE 6 Yield Tensile 2nd step aging Strength Strength time, hrs ALLOY # (MPa) (MPa) Elongation % @340 F. 6 263.5 318.0 16.0 16.0 7 299.5 352.0 13.0 16.0 8 305.0 354.0 14.0 4.0 8 309.0 361.0 15.0 4.0 8 310.0 362.5 15.0 16.0 9 311.0 356.0 11.0 4.0 9 304.5 355.0 13.0 16.0 18 304.5 351.0 14.5 4.0 18 297.0 345.0 14.0 16.0 19 293.5 341.0 16.0 4.0 19 270.0 324.5 16.0 16.0 20 299.0 342.0 18.0 4.0 20 264.0 316.0 18.0 16.0

As indicated by comparison of Tables 5 and 6, Al—Zn—Mg—Cu alloys having alloying ranges within the scope of the present invention have similar if not greater tensile properties than similar Al—Zn—Mg—Cu alloys having less than 0.5 wt. % Cu. For example, alloy composition number 3 having a Cu content of 0.85 wt. % provides a tensile yield strength value of 325 MPa, while alloy composition number 8 being of similar composition to alloy composition number 1 provides a similar tensile yield strength of about 310 MPa when similarly heat treated to an overaged condition including a second stage heat treatment of 340° F. for about 16 hours. The incorporation of Cu within the range of 0.5 wt. % to 1.2 wt. % has little to no disadvantageous effect on the tensile yield strength of the alloy, as illustrated by Tables 5 and 6, yet advantageously increases the alloy's stress corrosion cracking (SCC) resistance, as illustrated in Tables 2 and 3.

Table 7 includes the mechanical properties measured for Al—Zn—Mg—Cu alloys (alloy composition numbers 10-17) having a Zn+Mg content of 6.0 or greater.

TABLE 7 Yield Tensile 2nd step aging Strength Strength time, hrs ALLOY # (MPa) (MPa) Elongation % @340 F. 10 344.6 389.5 14.5 4.0 10 350.0 402.0 17.0 16.0 11 353.1 399.0 13.0 4.0 11 357.0 407.0 13.0 16.0 12 358.5 405.0 13.0 4.0 12 362.3 415.0 13.0 16.0 12 350.5 403.0 13.0 24.0 13 370.1 419.8 11.3 4.0 13 366.0 418.0 16.0 16.0 13 354.0 409.3 13.0 24.0 14 364.5 412.0 13.8 4.0 14 376.0 431.0 14.0 16.0 15 371.3 422.4 11.5 4.0 16 387.8 434.9 11.0 4.0 17 398.5 445.3 9.5 4.0

Referring to Tables 5 and 7, increasing the Mg+Zn content to 6.0 wt. % or greater provides increases the alloys tensile yield strength, but disadvantageously decreases the alloy's resistance to stress corrosion cracking (SCC), as indicated in Tables 2 and 4. As explained above increasing the Mg and Zn content in a manner that increases the Zn+Mg content to greater than 6.0 wt. % decreases the corrosion potential of the precipitate free zone (PFZ) zone relative to the Al matrix to a point that cannot be offset by the addition of increased Cu without producing an excess of constituent particles of AlFeCu at the grain boundary that decreases the alloys' fatigue resistance and toughness. Further, the increased Zn+Mg also produces higher amounts of MgZn2 at the grain boundary, which also disadvantageously reduces the alloy composition's resistance to stress corrosion cracking (SCC).

The tensile properties were also measured from automotive steering knuckles cast using Vacuum Riserless Casting (VRC)/Pressure Riserless Casting (PRC) methods and composed of Al—Zn—Mg—Cu aluminum alloys in accordance with the present invention and having greater than 0.5 wt. % Cu and up to and including 0.9 wt. % Cu. FIG. 3 illustrates a plot depicting the relationship between the Cu content of the alloy and the tensile yield strength (data line 52), ultimate tensile strength (data line 51) and elongation of the alloy (data line 50). Each casting was aged to an overaged condition using a two stage heat treatment including a first stage at 250° F. for 3 hrs and a second stage at 340° F. for 16 hrs. Increases in tensile yield strength 52 and ultimate tensile strength 51 were recorded in Al—Zn—Mg—Cu alloys with increasing Cu content from greater than 0.4 wt. % Cu to about 0.9 wt. % Cu.

General Conclusion

General corrosion (corrosion attack mode) was evaluated using ASTM G110 corrosion testing, which is the “Standard Practice for Evaluating Corrosion Resistance of Heat Treatable Aluminum Alloys by Immersion in Sodium Chloride+Hydrogen Peroxide Solution”.

Referring to FIGS. 4 a-4 c depicting photographs of tests specimen evaluated using ASTM G110 corrosion testing, wherein alloy composition numbers 1, 2, and 4 represent alloy compositions in accordance with the present invention, alloy composition numbers 8, 9, 18, and 19, represent comparative alloy compositions having less than 0.5 wt. % Cu, and alloy composition number 13 represents a comparative alloy composition having a Zn+Mg content of 6.0 wt. %. The test specimens were cast from a directionally solidified (DS) mold.

Similar to the evaluations for stress corrosion cracking (SCC) performance and mechanical performance, each test sample was treated to a two-step heat treatment, in which the first stage including keeping the heat treatment constant at 250° F. for 3 hours. Following the first stage, an aging stage was conducted, in which the furnace temperature was raised to 340° F. for soaking times ranging from 4 to 32 hours. The alloy reached peak strength at 4 hours at 340° F. Overaged conditions were investigated at 16, 24 and hours at 340° F.

In accordance with the procedures detailed in ASTM G110, the test specimens were immersed in a solution 3.5% NaCl+H₂O₂ for 24 hours. Once removed from the corrosive solution, the test specimens were investigated using an optical microscope to determine the mode of corrosion attack and depth of corrosive attack.

As depicted in FIGS. 4 a-4 c, test specimen composed of alloy compositions having a Cu content of greater than 0.5 wt. % (alloy composition numbers 8, 9, 13, and 19) are generally more susceptible to corrosive attack by pitting than test specimen having less than 0.5 wt. % Cu (alloy composition numbers 8, 9, 18 and 19). More specifically, the frequency and depth of corrosion sites increases with increasing Cu content.

The effect of the Cu content on the depth of corrosion further illustrated with reference to FIGS. 5 a-5 c. FIGS. 5 a-5 c depict graphs of the depth of corrosion in microns of test specimen evaluated by ASTM G110 corrosion testing, wherein the test specimen include greater than 0.5 wt. % Cu (alloy composition numbers 1, 2, and 4) in accordance with the present invention; and comparative examples having less than 0.5 wt. % Cu (alloy composition numbers 6, 7, 8, 9, and 18). The test specimens included castings from directionally solidified (DS) molds. The data plotted includes the maximum corrosion depth measured and an average of the depth for five of the deepest corrosion sites. As indicated by FIGS. 5 a-5 c, corrosion depth for alloy compositions having greater than 0.5 wt. % Cu is greater than the corrosion depth for alloy compositions having less than 0.5 wt. % Cu. Deeper corrosion depth was measured from cast surfaces, as opposed to machined surfaces, which was believed to result from microsegregation of Cu on the as cast surface of the DS castings.

Referring to FIGS. 5 a-5 c, the corrosion depth decreased with increasing aging time. Each of the test samples where aged using a two stage heat treatment, in which the first stage including keeping the heat treatment constant at 250° F. for 3 hours and the second stage included raising the furnace temperature to 340° F. for soaking times ranging from 4 or 16 hours. The alloy reached peak strength with a second stage heat treatment of about 4 hours at 340° F. Overaged conditions were investigated at 16 hours at 340° F. The degree of overaging effects the corrosion mode, wherein greater degrees of overaging in Al—Zn—Mg—Cu alloys in accordance with the present invention result in corrosive attack having a greater degree of pitting, as opposed to intergranular corrosion, and lesser degrees of overaging in Al—Zn—Mg—Cu alloys result in corrosive attack having a greater degree of intergranular corrosion, as opposed to pitting. Intergranular corrosion results in a greater corrosion depth than pitting.

Referring to FIGS. 6 a-6 c, the mode of corrosion was evaluated by sectioning the test samples and visually inspecting the samples cross section with an optical microscope. Referring to alloy composition #18 having a Cu content on the order of 0.25 wt. % in FIGS. 6 a and 6 b, when heat treated to peak strength conditions, i.e. a first stage heat treatment at 250° F. for 3 hours followed by a 4 hour second stage heat treatment at 340° F., the corrosion mode of the alloy composition may he characterized as pitting. Referring to FIG. 6 a, corrosion depth generally increases in cast surfaces of alloy compositions including 0.5 wt. % or greater Cu contents greater, such as alloy composition numbers 1 and 13 composed of 0.85 wt. % Cu, when compared to alloy compositions having less than 0.5 wt. % Cu, such as alloy composition number 18 having 0.25 wt. % Cu. Referring to FIGS. 6 b and 6 c, increasing the Cu content to 0.35 wt. % or 0.45 wt. %, such as alloy composition numbers 7, 8, 9 and 1, changes the mode of corrosion to at least partially being intergranular corrosion. The mode of corrosion may be entirely intergranular corrosion in Al—Zn—Mg—Cu compositions aged to peak strength (i.e. 2^(nd) step at 340° F. for 4 hours) and having greater than 0.5 wt. % Cu, such as alloy composition number 16 having a Cu content of 0.65 wt. %, as depicted in FIG. 6 c.

FIG. 6 c further illustrates at least one advantage of the present invention, in which a heat treatment is provided to overage the alloy resulting in coarsening of the fine precipitates at the grain boundary to provide a discontinuous distribution of large precipitates interrupted by Aluminum. As discussed above, Aluminum has a greater resistance to corrosion than the grain boundary precipitates. Therefore, the discontinuous distribution of large precipitates at the grain boundary results in a discontinuous mode of corrosion at the grain boundary, which advantageously increases the stress crack corrosion resistance (SCC) of the alloy.

The heat treatment to provide the overaged condition included two stages, in which the first stage included heating the alloy from room temperature to 250° F. within one hour. The second stage is aging the alloy to the overaged condition, wherein Table 4 includes data for aging at 340° F. for 4 hours (peak condition), 340° F. for 16 hours (overaged condition), and aging at 340° F. for 24 hours (overaged condition). Alloy composition #16, as depicted in FIG. 6 c, clearly illustrates that the heat treatment of the present invention, i.e. averaging for greater than four hours at 340° F. during the second stage of the heat treatment, advantageously converts the mode of corrosion from intergranular to pitting.

Castability

The castability of the Al—Zn—Mg—Cu alloy of the present invention having greater than 0.5 wt. % Cu was assessed using pencil probe for hot cracking index and spiral molds for fluidity. The hot cracking index is the smallest diameter of the central connection rod on the pencil probe casting that does not exhibit cracking, wherein the lower the value for the hot cracking index the better the hot cracking resistance of the alloy composition.

FIG. 7 illustrates the effect of the Cu content on the hot cracking index of an Al—Zn—Mg—Cu alloy, having 4.5 wt. % Zn, 0.09 wt. % Zr. and 1.8 wt. % Mg or 1.5 wt. % Mg. The Cu content hot cracking resistance of the Al—Zn—Mg—Cu alloy of the present invention is not substantially affected by the incorporation of Cu being greater than 0.5 wt. %.

FIG. 8 illustrates the effect of the Cu content on the fluidity of an Al—Zn—Mg—Cu alloy having 4.5 wt. % Zn, 0.09 wt. % Zr, and 1.8 wt. % Mg or 1.5 wt. % Mg. Cu had no appreciable effect on fluidity in Al—Zn—Mg—Cu compositions including 1.8 wt. % Mg, and provides a slight increase in fluidity in Al—Zn—Mg—Cu compositions including 1.5 wt. % fluidity.

While the present invention has been particularly shown and described with respect to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms of details may be made without departing form the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims. 

1. An aluminum alloy comprising: about 3.5 wt. % to about 5.5 wt. % Zn; about 1.0 wt. % to about 3.0 wt. % Mg; about 0.5 wt. % to about 1.2 wt. % Cu; less than about 1.0 wt. % Si; less than about 0.30 wt. % Mn; less than about 0.30 wt. % Fe; and incidental impurities.
 2. The aluminum alloy of claim 1 comprising a total Mg and Zn content less than or equal to 6.0 wt. %.
 3. The aluminum alloy of claim 1 further comprising at least one grain refiner selected from a group consisting of boron, carbon and combinations thereof.
 4. The aluminum alloy of claim 3, wherein said at least one grain refiner includes boride in a range from about 0.0025 wt. % to about 0.05 wt. %.
 5. The aluminum alloy of claim 3, wherein said at least one grain refiner includes carbide in a range from about 0.0025 wt. % to about 0.05 wt. %.
 6. The aluminum alloy of claim 1 further comprising at least one anti-grain growth agent selected from the group consisting of zirconium, scandium and combinations thereof.
 7. The aluminum alloy of claim 6, wherein said at least one anti-grain growth agent includes zirconium in a range below 0.2 wt. %.
 8. The aluminum alloy of claim 6, wherein said at least one anti-grain growth agent includes scandium in a range below 0.3 wt. %.
 9. The aluminum alloy of claim 1, wherein said zinc is at a concentration of about 4.2 wt. % to 4.8 wt. %.
 10. The aluminum alloy of claim 1, wherein said magnesium is at a concentration of about 1.2 wt. % to 1.8 wt. %.
 11. The aluminum alloy of claim 1, wherein said copper is at a concentration of about 0.6 wt. % to about 0.9 wt. %.
 12. The aluminum alloy of claim 11, wherein said copper is at a concentration of about 0.7 wt. % to 0.8 wt. %.
 13. The aluminum alloy of claim 1, wherein a concentration of iron in said alloy is less than about 0.3 wt. %.
 14. The aluminum alloy of claim 1, wherein a concentration of manganese in said alloy is less than about 0.3 wt. %.
 15. A method of making a shaped casting comprising: providing an aluminum melt comprising about 3.5 wt. % to about 5.5 wt. % Zn; about 1.0 wt. % to about 3.0 wt. % Mg; about 0.5 wt. % to about 1.2 wt. % Cu; less than about 1.0 wt. % Si; less than about 0.30 wt. % Mn; less than about 0.30 wt. % Fe; and incidental impurities; casting at least a portion of the melt into a mold to provide a shaped casting; and heat treating the shaped casting to an overaged condition.
 16. The method of claim 15 wherein, heat treating the shaped casting to the overaged condition further comprises: heating the shaped casting from about room temperature to about 250° F. (actually it is a range between 200 to 300° F.) within a time period of one hour; and aging the shaped casting at a temperature greater than about 340° F. for greater than about four hours.
 17. The method of claim 16, wherein the temperature of the aging of the shaped casting ranges from about 340° F. to about 380° F. for greater than four.
 18. The method of claim 16, wherein the aluminum melt further comprises a total Mg and Zn content of less than or equal to 6.0 wt. %. 